Paraquat sorption- and organic matter-induced modifications of soil spectral induced polarization (SIP) signals

Quantifying the capacity of soils to immobilize sorbing contaminants of concern relies on batch sorption experiments, typically performed at skewed solid-to-liquid ratios. The geophysical method spectral induced polarization (SIP) provides a powerful non-invasive monitoring alternative that can capture changes in soil electrical properties driven by contaminant sorption, yielding an approach whereby immobilization can be monitored in situ. Here, we present SIP signals obtained from a series of columns packed with a water saturated natural sandy-loam soil, with and without solid organic matter (SOM) amendment, contaminated with increasing concentrations of the herbicide paraquat. Our results highlight that soil polarization drops proportional to increasing amounts of sorbed paraquat in the SOM-free soil, exhibiting a Langmuir-type leveling-off behaviour. The addition of 8 percent-SOM yielded an increase in both the real (σ) and imaginary (σ) conductivity of the uncontaminated treatment, driven by the contribution of charged sites in the SOM. Further, SOM modified the dependence between σ and sorbed paraquat, likely driven by continued polarization within the polydisperse SOM with continued paraquat addition. However, the time constant (τ), derived using the Cole-Cole model, shed light on a saturation-type dependence governed by a drop in ion mobility with increasing sorption. Thus, aiding the interpretation of sorption-driven SIP signals.